The present invention relates to data communications and in particular to a system for measuring impairments in a data communication channel employing quadrature amplitude modulation.
It has long been known that the performance of amplitude modulation data transmission systems could be evaluated qualitatively by the subjective evaluation of eye diagrams. The use of and generation of such signal constellations or eye diagrams is well known and disclosed in the art. See for example U.S. Pat. No. 4,035,625. Indeed, many manufacturers provide a voltage or meter indication which is proportional to the eye scatter for use as a rough indication of receiver performance.
With the increased usage of quadrature amplitude modulated transmission systems, which has occurred in recent times, qualitative analysis of the signal constellation diagrams (sampled eye diagrams on a 2-dimensional plane) has been accepted as a means of evaluating the performance of the various types of QAM systems.
In FIGS. 1 and 2 there are shown respectively 8 and 4 point idealized signal constellations or eye diagrams. The horizontal axis of these diagrams may be referred to as the real, "X" or in-phase channel axis. The vertical axis may be referred to as the imaginary, "Y" or quadrature channel axis. FIG. 1 depicts an 8 point eye diagram obtained by combining the 4 point eye diagrams depicted in FIGS. 2A and 2B. 16 point eye diagrams are also obtainable as discussed in the above noted patent.
The eye diagrams are typically viewed by a technician or operator on an oscilloscope screen by modulating the horizontal axis with the X-signal and the vertical axis with the Y-signal.
Degradation of the communication channel manifests itself on the eye diagram in relatively well defined modes, the more common of which are depicted in FIGS. 3A-3D which, in its ideal form would correspond to FIG. 2A. Thus, if the communication medium is degraded by gaussian noise, the small clearly defined dots of the receiver eye diagram depicted in FIG. 2A would enlarge to roughly circular areas as a result of the noise having been added to the data signal. This is shown in FIG. 3A wherein the diameter of the circular areas is a measure of the noise. Phase jitter contributed by the communications media results in the clearly defined dots of FIG. 2A being displaced as an arc centered at the ideal points. This is depicted in FIG. 3B. Frequency offset and harmonic distortion cause the unique changes in the eye diagram as depicted in FIGS. 3C and 3D respectively.
As noted, since the various distortions manifest themselves in a unique fashion on the eye diagram, it would appear that a qualitative determination of the communications channel could be made by study of the eye diagram. In actual practice, however, the situation becomes complicated by the fact that the degrading influences can be cumulative and thus the clearly defined patterns depicted in FIGS. 3A-3D become distorted by the interaction of the combination of degrading factors. In addition, the eye diagram data does not lend itself to quantitative analysis of the condition of the communications channel primarily because the degrading effect in opposite quadrants tend to cancel each other out. As a result, heretofore, quantitative evaluation of the communication media has not been possible from the eye diagram.
In view of the above, it is the principal object of the present invention to provide an improved system for quantitatively evaluating data transmission over a communication media.
A further object is to provide such a system which operates in service while normal data is processed by the communications system and without interfering with the communication of the data.
A still further object is to provide such a system which may be applied to regular quadrature amplitude modulation data communication systems as well as to communication systems employing combined amplitude/phase modulation, phase shift keying, double side band modulation, etc.
Still further objects and advantages will become self-evident from the following description of the invention.